1. Field of the Invention
The present invention relates to a method for transferring isochronous data and an apparatus therefore, and more particularly, to a method for transferring variable isochronous data using the IEEE 1394 standard.
2. Description of the Related Art
It is possible to transfer data at a high speed by the IEEE 1394 serial bus. It is also possible to perform real time data transfer for a multimedia application by the IEEE 1394 bus. FIG. 1 shows the cycle configuration of the IEEE 1394 bus according to a conventional technology. In the IEEE 1394 bus, the maximum bandwidth of an isochronous transfer phase (100 μs) and the minimum bandwidth of an asynchronous transfer phase (25 μs) are specified in the P-1394 specification.
In general, isochronous transfer is used for real time data transfer and asynchronous transfer is used for data transfer for various commands such as reproduction, rewind, pause, and change of channels. Therefore, the maximum bandwidth of an entire isochronous cycle (125 μs) is specified.
FIG. 2 shows the gap timing of the IEEE 1394 bus according to a conventional technology. Referring to FIG. 2, according to the gap timing of the IEEE 1394 bus, isochronous packets, to which an arbitration bit, a data prefix bit, and a data end bit are added. are transferred with isochronous gaps interleaved therebetween. At this time, an acknowledge packet which indicates acknowledge to request can be transferred after an acknowledge gap.
In order to explain the isochronous transfer of a packet having the above gap timing, the main steps of a method of assigning isochronous bandwidth in the IEEE 1394 bus according to the conventional technology are shown in FIG. 3. A node which manages the IEEE 1394 bus can be an isochronous resource manager (IRM), according to a control situation. The node which becomes the IRM includes a bandwidth available (BA) register which manages isochronous resources, in a control and status register (CSR) defined by the IEEE 1394 bus standard. Other nodes in the IEEE 1394 bus read the value of the BA register of the IRM in order to perform an isochronous transfer. Then, it is determined whether there is bandwidth to be assigned and the isochronous transfer is performed when there is bandwidth to be assigned. At this time, the entire available bandwidth is generally assigned in a process of assigning the bandwidth. However, the entire assigned bandwidth is not used in a real isochronous transfer process. Also, in the IEEE 1394 bus, the bandwidth is assigned, assuming that transfer, according to a corresponding application of a node, is in the form of a constant bit rate (CBR) of an asynchronous transfer mode (ATM). Therefore, when the transfer is in the form of a variable bit rate, residual bandwidth results. For example, applications using the IEC-61833 standard are given much weight in the IEEE 1394 applications. In the isochronous transfer method for audio/video devices according to the IEC-61833 standard, it is specified that only a header is transferred to a corresponding isochronous cycle when there is no data to be transferred. As a result, in such a case, the entire assigned bandwidth is not used, and there are cases where only necessary data is transferred only when necessary in order to prevent the repeated transfer of data and to facilitate the controlling of a buffer. Thus, pertinent, the assigned bandwidth is not used at all.
FIG. 4 shows the relationship between the assignment of bandwidth and the use of bandwidth in the IEEE 1394 bus according to the conventional technology. Referring to FIG. 4, the horizontal axis shows the degree of the use of an isochronous channel and the vertical axis shows bandwidth available. Since the assigned bandwidth increases as the use of the channel increases, free bandwidth is relatively reduced. However, since the entire assigned bandwidth is not used during the real isochronous transfer as mentioned above, residual bandwidth is generated as shown in FIG. 4. Therefore, in the method for transferring the isochronous data according to the conventional technology, the bandwidth is not effectively used. Also, no more bandwidth can be assigned although there is bandwidth which can be used after a break point.
It will now be explained how isochronous and asynchronous transfer modes supported in the 1394 bus are distinguished from each other and how the channels and bandwidth used for the isochronous transfer are managed. Also, the transfer service of the isochronous transfer mode of the 1394 bus will be simply compared with the transfer service of the asynchronous transfer mode (ATM).
The 1394 bus provides two transfer modes, namely, an isochronous transfer mode and an asynchronous transfer mode. In isochronous transfer, one packet can be transferred every isochronous cycle of 125 μs to provide real-time transfer. Meanwhile, in the asynchronous transfer, which is a transfer mode where data transfer is guaranteed, the data transfer is controlled by a fairness interval where all connected modes can transfer data only once in the interval. Isochronous and asynchronous data are transferred for one isochronous cycle, which is started by the cycle start packet transferred from a root node. The isochronous transfer is prior to the asynchronous transfer. However, in order to prevent a situation where the asynchronous transfer can not be performed at all due to the isochronous transfer, only 80% of the entire cycle can be used for the isochronous transfer. Namely, at least 20% of the entire cycle can be used for the asynchronous transfer. If the isochronous transfer does not exist, the entire cycle can be used for the asynchronous transfer.
TABLE 1Transfer classificationAsynchronousStreamCharacteristicsIsochronousRead/WriteBroadcast writechannelTimeYesNoNoNoguaranteedArrivalNoYesNoNoguaranteedBroadcastYesNoYesYesChannelNot specifiedNot concernedYessharedAs shown in Table 1, asynchronous read and write, asynchronous broadcast write, and asynchronous stream channel which is newly defined in the P1394a specification can be used in the ATM. Here, guaranteed delivery of data through an identification process can be performed only by the asynchronous read and write.
Channels and bandwidth of the IEEE 1394 bus are respectively assigned using the values of two registers, CHANNELS_AVAILABLE (CA) and BANDWIDTH_AVAILABLE (BA) of the IRM which are determined after resetting the bus. The channels and bandwidth must be previously assigned from the IRM to an application wherein isochronous transfer of data is desired to be performed using asynchronous lock transaction in order to maintain integrity of resource information. Accordingly, a concerned node can transfer data of an assigned bandwidth using the assigned channels and bandwidth regardless of whether other nodes use the bus. Namely, it can be considered as using an additional physical channel having a bandwidth as large as the assigned bandwidth. Since the assignment unit of the isochronous bandwidth is calculated using the time taken to send 1 quadlet of 4 bytes in S1600 (1.6Gbps) specification, the initial value of the BA register becomes 4915. The value four times the value read from the BA must be assigned as a bandwidth assignment value in S400 specification. In the case of the asynchronous stream service defined in the P1394a standard, channels are used like in the isochronous service, however, the bandwidth is not assigned. Therefore, the asynchronous stream service cannot be used for real-time transfer.
The largest difference between the 1394 transfer and the ATM transfer lies in the sizes of transfer units. A packet having a variable size where the maximum size of the transfer unit is determined in proportion to transfer speed is used in the 1394 transfer. Meanwhile, a cell having a fixed size of 53 bytes is used as the transfer unit for high speed processing in the ATM transfer. Another difference lies in that a transfer line to which a switch is connected can be controlled in the ATM transfer and that the use of the bus by other nodes cannot be directly controlled in the 1394 transfer. It is difficult to effectively use the bandwidth, maintaining uniform transfer ability and characteristics when the 1394 transfer is used together with the ATM transfer due to basic differences. Therefore, a method of decreasing such differences must be provided. The IEEE 1394 specification is similar to the Ethernet, which is widely used as a LAN, excluding the isochronous transfer function. The transfer characteristic of the 1394 transfer is simply compared with the transfer characteristic of the ATM transfer in Table 2.
TABLE 2Transfer classificationIEEE 1394ATMPhysical mediumShared (multiplePrivate/Switching (pointaccess)to point)AddressingOperation node IDFixed address(IEEE-1212)Data unitVariable length packetFixed cell (53 bytes)Transfer rate controlNon-connectionConnection (VPC andVCC)FairnessPacket size basedCell rate basedbandwidthbandwidthReal time transferHN, DAN, and CANLAN, MAN, and WAN
The QOS characteristic of the ATM transfer is shown in Table 3.
TABLE 3QOSAvailableConstant bitVariable bitbit rateUnspecifiedCharacteristicsrate (CBR)rate (VBR)(ABR)bit rate (UBR)CDV (cellYesYesNoNodelay variable)CTD (cellNoYesNoNotransfer delay)CLR (cell lossYesYesYesNoratio)Variables forNot specifiedPCR andPCR andNon (performpreparingSCRMCRoptimal effort)bandwidth
As shown in Table 3, there are four services, CBR, VBR, ABR, and UBR in the ATM transfer service as shown in Table 3. Real-time transfer can be performed by the services, CBR and VBR among these services.
However, the isochronous transfer of the IEEE 1394 corresponds to the CBR service for processing data having a specific bit rate in view of the ATM. The asynchronous transfer, where delivery by the best-effort is assumed, is similar to the UBR service of the ATM, excluding the guaranteed delivery characteristic. For example, the bandwidth of the ATM is first assigned to the CBR and VBR services and the remaining bandwidth is assigned to the ABR and UBR services. The ABR service has the characteristics of both the VBR and UBR services. As a result, according to the conventional 1394 standard, the VBR and ABR services of the ATM are not properly supported.